The present invention generally relates to apparatus and methods for submerged processing of a work surface and particularly relates to apparatus and methods for excluding a liquid from a work surface having a groove, thereby affording a local dry area along the groove and enabling submerged operation of the processing apparatus such as a welding torch, heating device or stress-relieving device along the groove.
Submerged or underwater processing applications such as welding, thermal stressing and the like require a local dry area around the processing head in order that water can be excluded from the work surface to be processed. For example, in submerged welding, the water must be excluded from the molten metal and nearby heated zone to prevent excessive oxidation, premature cooling and other defects. Inert gas is typically used to displace the water locally around the welding head and to provide a chemically inert atmosphere for the molten metal pool. The work surfaces in many underwater applications, such as in a nuclear reactor pressure vessel which has been in service or offshore petroleum or marine vessel activities such as construction, maintenance or repair, are generally not smooth or regular. For example, it is difficult to exclude water from within a groove formed by machining out a crack in the surface of an underwater component, especially after it has been partially or completely filled and then partially excavated for interim repair. A similar geometry-related problem exists when attempting to exclude water from a groove formed by edge preparation between two parts being joined.
Existing designs for water or other liquid exclusion devices for underwater applications have three basic principles of operation: (1) mechanically sealing the gap between the work surface and the application head, e.g., in a welding environment, a cup-shaped gas-filled component around the torch end; (2) flowing gas across the relatively small controlled-width gap between the work surface and the application head; or (3) providing a diverging water/gas cone or cylinder flow across a controlled gap to displace water within the contact area of the cone or cylinder against the work surface. Design variations combining these principles include a gas-permeable compliant seal for multiple concentric flowing water and gas cones/cylinders. The designs relying on a compliant seal have an inherently limited practical working range because an elastic element is deformed to provide compliance and this element has a limited strain range (before it deforms plastically or is fully compressed), as well as a significantly increasing force requirement for increasing displacement which must be overcome by applicator head manipulation to maintain the desired position along the contoured surface. The force requirement and high displacements may be reduced somewhat by employing thinner or softer deflecting seal elements. However, these thinner elements are increasingly prone to mechanical damage due to inadvertent overloading during use or by tearing during handling operations or while sliding over work surface disparities and discontinuities.
Designs relying on positive water or gas flow through a gap have the limitation that local contour changes or tilting of the applicator head typically generate a differential gap, resulting in the expected differential gas flow around the perimeter of the gap. When the gap is greater in one area, the flow rate and flow velocity of gases, particularly in the case of welding, also becomes greater at the expense of the flow rate and velocity in the remaining areas of the perimeter having a lesser gap. As the flow is reduced in the areas having a lesser gap, the flow rate falls below the minimum required to hold back the water without surging of the water/gas interface or, catastrophically, reverse flow of the water toward the dry welding or process zone within the applicator head housing occurs.
Existing water exclusion devices, however, are particularly and inherently inefficient for use over grooves in the surface of substrates since there is no provision either to limit the flow clearance between the device and the lowest portion of the groove as it is filled with the weld deposit. In the case of xe2x80x9cflow curtainxe2x80x9d types of devices, there is also no provision to provide a differential or separate flow over the groove which is greater than the needed amount away from the groove. As a result, the ability of known devices to exclude water using an internal flow rate below that which disturbs the welding process is limited at best or is ineffective at worst in the case of work surfaces having grooves. The effectiveness of these devices to exclude water from within deep grooves is limited. Even when used within the depth limit of the ability of these devices to exclude water in grooves, they are wasteful of purge fluid, typically high-purity inert gas, since the flow occurring in areas away from the groove is greater than required in the groove just to have sufficient flow over the groove itself. In the worst case, they are totally ineffective to exclude water from very deep grooves. Accordingly, there is a need to reliably exclude water from within shallow or deep grooves formed in submerged substrate materials without excessively high gas flow from within the device.
In accordance with a preferred embodiment of the invention, there is provided a liquid exclusion apparatus surrounding an applicator head such as a welding torch or material processing device which has significant capability to reliably follow work surface contour changes without allowing liquid such as water to enter the dry area around the applicator head or work surface being processed. To accomplish the foregoing, the apparatus of the present invention includes a closed housing having an opening through a face of the housing for juxtaposition to the work surface and through which an applicator head, such as a welding torch or material processing device, may perform its intended function. In one preferred form hereof, the applicator head is carried by a slidable plate along another face of the housing opposite the opening such that the applicator head can be located at laterally different positions relative to the housing. The margin of the applicator opening about the housing is provided with a seal, for example, a wire mesh rope-type seal, to minimize or preclude water entry into the housing. In accordance with a preferred embodiment of the present invention, the housing is also provided with a pair of slidable gates. Each gate has a surface for extension from the side of the housing containing the applicator opening onto the work surface contour, e.g., a groove. The gate is biased by one or more springs to engage the sealing edge in the groove as the housing is moved along the work surface to exclude water from within the housing. With the bias applied to the sliding gate, the gate seal travels along the bottom of the groove forming a sufficient mechanical seal as the work surface contour of the groove changes. Processor purge gas flowing inside the exclusion device facing the work also displaces water from within the device through the controlled clearance, maintaining the work area water-free.
In an alternate form of the present invention, a pivoted or rotatable gate mounted to the housing is deployed. The pivoted gate, similar to the sliding gate, has an edge with sealing material for engaging and sealing against the contoured work surface, such as the groove. The rotating gate has the advantage of reducing the sliding friction force between the work surface and the contacting edge of the gate during weld progression along the groove since the angle between the gate and the work surface may be less than 90xc2x0 at the beginning of the weld in the groove bottom and is reduced further as the groove is filled. As in the case of the sliding gate, purge gas is prevented from escaping excessively from the edges of the gate due to the containment action of the gate frame. Each pivoted gate is also movable toward and away from the work surface in addition to its pivotal movement.
Moreover, the present invention can successfully travel over the sloped beginning and end of grooves that do not extend around the full perimeter of the work surface to form an endless closed path. With the present invention, high gas flows typical of existing methods and devices which disturb the welding process by chilling or displacing the molten metal pool or are wasteful of the inert gas commonly used to provide the liquid/gas boundary at the perimeter of the device are avoided. That is, low gas flows may be used in the present invention which do not disturb the process being applied yet are sufficiently high to effectively displace the surrounding liquid in the gap between the exclusion device and the work surface. The present invention also prevents an excess of liquid vapor at the water liquid/gas interface from entering the weld zone which would otherwise likely react with the hot or liquid metal and cause weld defects.
In a preferred embodiment according to the present invention, there is provided apparatus for processing a work surface having a groove, comprising a closed housing having an opening and movable relative to the work surface in the direction of the groove, a working head carried by the housing for processing the surface through the opening, a seal about margins of the housing for sealing against the work surface about the opening and at least one gate having a sealing edge and carried by the housing for movement into a position extending from the housing for engaging the groove and means for biasing the gate for movement into the extended position.
In a further preferred embodiment according to the present invention, there is provided in an apparatus for processing a submerged work surface having a closed housing, an opening, a working head within the housing and a pair of gates carried by the housing mounted for movement relative to the housing, a method for excluding fluid from the work surface, comprising the steps of extending the gates substantially independently of one another relative to the housing, enabling seals along edges of the gates to engage or lie in close proximity to the submerged groove in an extended position of the gates, advancing the housing along the work surface with the edges of the gates following the contour of the groove of the work surface and movable independently of one another to substantially exclude fluid from the work surface exposed to the working head within the housing and operating the working head to process the work surface through the opening as the housing is advanced along the work surface.